The present invention relates to optical communications systems. More particularly, the present invention relates to optical communications systems for providing an increased degree of security in the transmission of communications information.
In prior art optical fiber communication systems a communications signal modulates a device such as a light emitting diode or semiconductor laser coupled to a thread-like optical fiber transmission medium which may be of the order of kilometers in length. At the other end of the optical fiber an optical receiver including a photodiode and amplifier apparatus is provided to retrieve the signal information.
Such a prior art optical fiber communications system suffers from the disadvantage that it can be readily tapped in analogy with wiretapping. For example, an unauthorized person can mechanically introduce ripples or bends in an intermediate section of the optical fiber, place an optical coupling medium around the fiber and locate a photodetector to receive a tiny portion of the light carrying the transmitted information, thereby achieving an optical tap. It is apparent that such a communications system is not fully advantageous for many applications requiring a high degree of communications security.
Likewise, a variety of optical systems utilizing non-microscopic beams of light and beams of light in the open air suffer from similar forms of insecurity and additional problems of inconvenience and limitations on rate of communication.
Accordingly, it is an object of the present invention to provide a method and system and optical communications components therefor to overcome the tappability and other problems in prior art optical communications systems.
The problem of rendering communications more secure is one which characteristically does not admit of total solutions in that provisions for increased security in systems generally increase the cost and difficulty of unauthorized access but do not render it impossible. However, provisions for security can effectively preclude unauthorized receptions except by the most determined and well financed intruder.
Where the transmission system consists of a subscriber station at subscriber controlled premises, a transmission line, and a telephone company central office, the rational intruder will attempt to violate the security of the "weakest link" in the system. That is, the intruder can be expected to strike where a "perceived exposure" criterion, including financial expense and risk of detection factors, is least. When provisions for security are being made at the subscriber premises and the telephone company central office, it is not unreasonable to expect an intruder to attempt to violate the security of the transmission line. Thus, increasing the security of the transmission line can increase the security of the transmission system as a whole. Similar considerations apply where both ends of the transmission system are controlled by the same entity, of course.
Consequently, it is an object of the present invention to provide greater security for information transmitted in an optical fiber communication system by substantially increasing the cost, difficulty and risk of detection of unauthorized reception from an optical fiber transmission line used therein.